returns a value of x, cyclically-modulated by M

xModByM.js

const xModByM = (x, M) => ((x % M) + M) % M;

returns the ACTUAL modulus value of a number; that is to say, instead of negative numbers returning reverse-mod-negative values - as
happens with %= or any other modulus use - it converts the ENTIRE number-line into a cyclical universe of values from 0 - M

if you are at, say, the x-coordinate 3px, and you want to move_negative_ twenty pixels through a cyclical universe that is a hundred pixels
wide, compare these two results:

console.log(
    (3 - 20) % 100,     // produces -17
    xModByM(3-20, 100)  // produces  83
)